The purity of human serum albumin has been studied. Commercial preparations differ in their fluorescence spectra, quantum yields, and fluorescence decay parameters. Removal of impurities alters these properties. These findings are relevant to the use of human albumin in optical studies. Fluorescence depolarization measurements have commonly been made on proteins to determine their size and shape. We have investigated the use of the proteins' native ultraviolet fluorescence for this purpose instead of the fluorescence of attached dyes. The results suggest that the use of dyes is not always necessary, reasonable results can be obtained with the intrinsic fluorescence, although caution must be taken to factor out the independent rotary motion of the indole groups.